Recognition: The least weasel is the smallest member of the Order Carnivora worldwide. It has typical weasel features, including a long, slender body, short legs, a long neck, a flat head, large black eyes, rounded ears, and long vibrissae. Summer pelage is chocolate brown dorsally and white ventrally. Dorsal winter pelage in Kansas is brown like summer pelage in some specimens, mottled brown and white in some others, and completely white in others. The tail does not have a black tip. The dental formula is incisors 3/3, canine 1/1, premolars 3/3, molars 1/2.

Confusing Species: The only species in Kansas with which the least weasel might be confused is the long-tailed weasel. From that species, the least weasel easily can be distinguished by its much smaller size, relatively shorter tail, and the lack of a black tip on its tail.

Distribution:The least weasel has a circumboreal distribution, occurring in both the Old and New Worlds. In North America, it ranges southward across Alaska and much of Canada onto the Great Plains and the Appalachian Mountains. On the Great Plains, it was not known to occur south of Nebraska until 1965, when a specimen was trapped in Marshall County. The species continued dispersing rapidly across Kansas and was documented as far south as northeastern Oklahoma in 1988. The current distribution of the least weasel in Kansas potentially includes all areas of the state except the Southwest.
County Breakdown: County Name (# occurrences)
Barton (2), Brown (2), Butler (1), Clay (2), Cloud (2), Coffey (1), Ellis (13), Ellsworth (1), Harvey (1), Jackson (3), Jefferson (1), Jewell (9), Lincoln (2), Marion (11), Marshall (2), McPherson (3), Mitchell (1), Ness (1), Osborne (1), Ottawa (2), Phillips (1), Pottawatomie (2), Reno (1), Republic (4), Riley (5), Rooks (3), Rush (2), Russell (2), Saline (5), Sedgwick (1), Smith (7)

Reproduction: Breeding occurs throughout the year but is concentrated in spring and late summer. Courtship is initiated when an estrous female allows a male to approach. After vigorous struggling and fighting, the male grabs the female by the scruff of the neck and holds her down. Copulation is repeated over several days. Ovulation is induced by repeated copulation. Gestation lasts 34 to 37 days, and the number of young per litter typically is 4 or 5. Females usually produce 2 or 3 litters per year. Newborn are blind and naken but are covered with a fine, white coat of fur by about 4 days of age. Their adult fur develops between 2 and 3 weeks after birth. Their teeth erupt at 2 weeks, and their permanent dentition develops between 30 and 49 days. The young begin to eat solid food at about 3 weeks. Their eyes open between 26 to 32 days. Shortly after their eyes open, they begin to leave the den. At this time the female teaches the young to kill prey, and by 6 or 7 weeks after birth they are proficient hunters. At 8 weeks of age, the young males are larger than the females. The young reach adult size between 12 to 14 weeks and probably disperse at about that time. Least weasels become sexually mature at approximately 4 months of age. The male plays no role in caring for the young.

Behavior: The least weasel is found most commonly in meadows and grasslands, reaching its greatest abundance in marshy areas, and is least common in dense forest lacking ground dover. At the periphery of its range in Kansas, this species seemingly occurs in all available habitats, including agricultural land. Active both night and day throughout the year, least weasels investigate holes and burrows searching for prey. Many more prey may be killed than can be eaten, with the remainder cached for later use. A least weasel feeds 5-10 times per day, and it leaves uneaten remains in burrows and other cache locations. Many burrows used temporarily by least weasels previously were used by the prey animals. The least weasel experiences pronounced population irruptions during periods of high prey abundance, at which time dispersal may occur.

Food Habits: The least weasel is a voraceous predator that specializes on mice and other small mammals but will prey on almost any animal that the weasel is big enough to kill (in some instances, up to the size of rabbits). It has been estimated that least weasels require approximately half their body weight in food per day.

Predators and Defense: Least weasels are preyed upon by larger predators, including raptors, snakes, and larger carnivores, including foxes, bobcats, long-tailed weasels, and domestic dogs and cats. Population turnover is rapid, and longevity in the wild seldom exceeds 3 years (as compared to 10 years in captivity).

Taxonomy: The subspecies of least weasel that occurs in Kansas is Mustela nivalis campestris.

Fossil Record: The ancestry of Mustela nivalis can be traced back to an extinct species that lived in Europe in the late Pliocene and early Pleistocene. Mustela nivalis is common in fossil deposits in Europe beginning in the middle Pleistocene. The species emigrated into North America in the late Pleistocene.

Remarks: Because it is small and secretive the least weasel is rarely found throughout its range. Its presence in Kansas has been suspected for many years, but the first specimen from the state was not collected until March 1964 near Marysville in Marshall County. Longevity of this small mammal is one or two years in the wild.